1. Field of the Invention
This invention relates to the field of the detection of carbon monoxide. More particularly, it pertains to the use of infrared absorption spectroscopy for such detection. A method of detection and measuring proposed hereinafter is simple, inexpensive, accurate and allows for the measurements to be conducted in the field, outside of a laboratory environment.
2. Description of the Related Art
One of the most important technological processes is the production of hydrogen on site, via a reformation process involving the reaction of water with methanol. Among other applications, this process is used, for instance, in the development of fuel cells where hydrogen so produced serves as fuel. The reformation process not only produces a fuel cell feed stream containing hydrogen, but also such by-products as carbon dioxide and small amounts of carbon monoxide.
The carbon monoxide constituent has the effect of poisoning the fuel cell at levels as low as 10 parts per million. Carbon monoxide is also known to be a harmful by-product, even when present in very small concentrations, in other processes and applications. Therefore, a method and apparatus for measuring and/or monitoring the concentration of carbon monoxide at levels of about 10 parts per million is desired.
Presently, the only reliable technique for sensing carbon monoxide at these levels is by infrared absorption spectroscopy. However, to resolve species at very low levels, the intrinsic drift of the instrument and the interference from other species present in large concentrations (such as, in this case, carbon dioxide which can occur in the amount of 18% by volume) or water present in large concentrations must be eliminated or compensated for during the measurement.
Currently, to compensate for the drift, the instrument is periodically put through a zeroing procedure using an internal standard as the zero reference material. See, e.g., N. Colthup, et. al., Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 1964, pp. 74–77. In many cases, the standard reference gas used is ambient air.
While air provides a reference absent of carbon monoxide, it does not address the compensation for the interfering gases. An additional problem is that the concentration of the interfering gases is dynamic, thereby not allowing a fixed compensation to be built into the measurement system.
The zeroing procedure involves a technique of subtracting a reference gas absorption spectrum from a sample gas absorption spectrum to obtain a third spectrum where the contribution of species common to both gases has been eliminated. Such technique is known in the prior art. See, e.g., Model 6600 Miniature Automotive Gas Analyzer, Andros Incorporated's Product Manual.
In a laboratory environment, the reference process typically consists of performing a calibration measurement with a gas mixture that has a known concentration of the species to be measured, and another measurement with a gas mixture that does not contain the species. The gas mixture that does not contain the species is used to set the zero point for the instrument, and usually consists of room air. The reference gas that contains the species to be measured is preferably similar in composition to the sample gas mixture, such that potential interference from several gas species absorbing in similar wavelength regions is accounted for during the measurement.
In the field, or non-laboratory environment, periodic calibration of the instrument with two separate gases is typically not practicable. This two-reference gas calibration technique refers to a method by which the zero point and the span of the infrared instrument are calibrated. The zero point refers to the level whereby the instrument decides that the concentration of a measured gas is zero. The span refers to calibrating the instrument with a specific concentration of the gas species to be measured.
An example of the use of two-reference gas calibration technique would be exposing the instrument to a gas containing 100 parts per million (ppm) carbon monoxide, and then referencing future measurements to the amount of signal obtained at each level.
At best, an initial calibration with the two gases is performed, followed by periodic zero point references with room air. This technique is found to be somewhat useful in preventing large measurement errors introduced by baseline or zero point drift, but does not address errors introduced by the dynamic concentration of interfering species in the sample gas. When a high measurement accuracy (±5–10 ppm) is required in the under 100 parts per million range for a given species, the measurement error introduced by the interfering species must be somehow compensated for during the measurement.
However, no known prior art which involves the use of infrared absorption spectroscopy coupled with the zeroing technique allows for accurate detection of carbon monoxide at the very low concentrations mentioned above, especially for outside-of-the-laboratory use.
As will be subsequently discussed, the method proposed in this invention utilizes a catalyst to remove carbon monoxide from the feed stream, via its oxidation to carbon dioxide, with the subsequent use of the remaining feed stream gas as a zero reference. A catalyst is, therefore, needed that operates to efficiently remove carbon monoxide from the feed stream at low temperatures, namely as low as the normal operating temperature of the system (about 80° C.).
Previously developed catalysts for carbon monoxide oxidation include a commercially available, manganese oxide and copper oxide-based Hopcalite, and platinum or palladium-supported structures on oxide hosts. These catalysts depend on the presence of oxygen to form carbon dioxide from carbon monoxide, and require temperatures in the range of 150–350° C. to achieve high efficiency.
Furthermore, these catalysts exhibit a low degree of selectivity towards carbon monoxide oxidation in the presence of hydrogen. Such use of platinum catalysts for conversion of carbon monoxide into carbon dioxide is described, for example, in The Mechanism of the Catalytic Action of Platinum in the Reactions 2CO+O2=2CO2 and 2H2+O2=2H2O, Transactions of the Faraday Society, vol. XVII, Part 3, 621 (1921).
Other known catalysts that require only water to form carbon dioxide from carbon monoxide include iron oxide structurally in combination with chromium oxide, and copper/zinc oxide/aluminum oxide formulations. The iron oxide-chromium oxide catalyst, however, requires temperatures in the range of 300–450° C. and pressures exceeding 2.5 MPa. The copper/zinc oxide/aluminum oxide catalyst also requires temperatures higher than 110° C. See, e.g., H. Sakurai, et. al., Low Temperature Water-Gas Shift Reaction Over Gold Deposited on TiO2, Chem. Commun., p. 271 (1997).
In view of the foregoing, there is a need for a simple, inexpensive and accurate method for detection of carbon monoxide in an environment, including a fuel cell environment, which method would allow for the measurements to be conducted in the field.
The present invention proposes such method based on the periodic zeroing routine of the instrument, but improves upon the compensation process by using the actual feed stream gas, from which carbon monoxide has been removed, as the zero reference. With this technique, accurate detection of carbon monoxide in the 10 parts per million range can be realized.
Furthermore, there is a need for a catalyst which would allow such new method to be realized. Certain important feed streams operate at temperatures as low as about 80° C. and contain an abundance of water and hydrogen, with very small amounts of oxygen. It is therefore important that the catalyst be able to operate at low temperatures and be specific towards carbon monoxide oxidation when either oxygen or water is the available oxidizing agent. None of the existing catalysts mentioned above is satisfactory. New catalysts are needed which have this ability, and therefore offer an advantage over the other stated catalysts. Such catalysts are also taught in this invention.